The Am is called the relative minor of C major and is the main minor chord of the key. Hear how the minor chord will give a melancholy sound as opposed to the bright sound of the Major chord. If the principle chord is Am is said to be in the key of Am. It’s notes will be the same as C major but will start on A instead of C.
So we find that the I, IV and V chord of any key, minor or major contain the main chords of that key. As most popular music is written using these chords we will start with various keys that use simple open chords. The following examples have been selected because they use simple chords and are easy for a beginner to get started.
D minor = Dm, Gm and A minor
It is very beneficial to warm up and stretch before you start playing. This will give greater stretch and stamina but has the best benefit in keeping you able to play well in the long-term. I found this excellent tutorial on warming up.